The present invention generally relates to rotary hammers and, more particularly, to a pneumatic drive system for a rotary hammer.
In general, rotary hammers operate to impart both rotational, drilling movement and axial, hammering movement on a tool bit. In this regard, rotary hammers include both a rotary drive system and an axial drive system. A typical rotary drive system includes a gear and pinion assembly which imparts rotational movement on a spindle and on the bit. One type of axial drive system is a ratchet-type drive system which drives the bit through engagement of complementary ratchet surfaces. Another axial drive system includes a pneumatic drive system which uses an axially reciprocating piston to drive the bit.
A prior art pneumatic rotary hammer 200 is partially shown in FIGS. 5, 6, and 6A. The hammer 200 includes a rotary drive system 204 (partially shown) which drives a tool bit 208 for rotational drilling movement about an axis 212, and a pneumatic drive system 216 which is operable to impart axial, hammering movement on the tool bit 208. The pneumatic drive system 216 is selectively engageable and includes a cylindrical barrel 220, a reciprocating piston 224, a ram 228, and a striker 232. The piston 224 reciprocates within the hollow portion of the barrel 220 to form a piston and cylinder assembly. A seal 236 is supported on the piston 224 and forms a seal between the piston 224 and the barrel 220. The ram 228 is supported within the barrel 220 for axial movement relative to the barrel 220. A seal 240 is supported on the ram 228 to form a seal between the ram 228 and the barrel 220. The ram 228 is engageable with the rearward end of the striker 232 to cause hammering. The forward end of the striker 232 engages the bit 208.
Idle ports 244 are formed circumferentially in the sidewall of the barrel 220. When the idle ports 244 are open to the atmosphere (FIG. 5), air flows into and out of the space between the piston 224 and the ram 228 through the idle ports 244. When the idle ports 244 are closed (FIGS. 6 and 6A), air cannot flow into and out of the space between the piston 224 and the ram 228. When the piston 224 reciprocates with the idle port 244 closed, a vacuum is formed in this space. A forward port 248 is also formed in the sidewall of the barrel 220. Air flows into and out of the space between the ram 228 and the striker 232 through the forward port 248.
The hammer 200 has an idle mode (FIG. 5) and a hammer mode (FIGS. 6 and 6A). In the idle mode, although the piston 224 reciprocates, the pneumatic drive system 216 does not operate to impart axial, hammering movement on the bit 208. More specifically, the idle mode of the hammer 200 corresponds with the ram 228 being in the forward position (FIG. 5). In this position, the idle ports 244 are open so that air moves into and out of the space between the piston 224 and the ram 228 as the piston 224 reciprocates.
To engage the hammer mode, the operator pushes the bit 208 against the workpiece forcing the striker 232 to move rearward. The striker 232 pushes the ram 228 rearward so that the seal 240 on the ram 228 crosses and closes the idle ports 244 (see FIGS. 6 and 6A). This seal allows a vacuum to be formed in the space between the piston 224 and the ram 228. As the piston 224 moves rearwardly, the ram 228 will follow due to the vacuum (FIG. 6). On the forward stroke of the piston 224, the ram 228 is forced forward and slams into the striker 232 (FIG. 6A). The striker 232, in turn, slams into the bit 208 causing a hammering motion. This hammering cycle continues as the piston 224 reciprocates and as long as the idle port 244 is closed.
When the bit 208 is withdrawn from the workpiece, the ram 228 will move back to the forward position so that the idle ports 244 are uncovered (FIG. 5). When this occurs, the seal 240 on the ram 228 again crosses the idle ports 244. In the typical prior art rotary hammer 200, the operator disengages the hammering after only relatively few (e.g. about ten) hammering cycles. The hammer mode must be re-engaged by again moving the ram 228 and the seal 240 over the idle ports 244.
The movement of the seal 240 across the idle port 244 causes the seal 240 to wear. If the seal 240 fails, a vacuum will not be formed in the space between the piston 224 and the ram 228, and the pneumatic drive system 216 will not operate as effectively. The seal 240 must be replaced for the hammer 200 to operate at its full capacity. If the seal 240 fails on the rearward stroke of the piston 224, the ram 228 can slam into the piston 224, causing severe damage to the piston 224. In this case, the complete pneumatic drive system 216 may need to be replaced. To prevent the seal 240 from wearing out and failing, another part, such as a conductive brush (not shown), is commonly designed to wear out before the seal 240. Once the conductive brush wears out, the electric motor will not operate. At this point, the user normally returns the hammer to the manufacturer for a complete overhaul at which time the seal 240 is also replaced.